In certain railway operations, such as, in the sorting of railway vehicles in hump type of classification or marshalling yards in accordance with consist and destination, it is necessary to control the speed of the free rolling vehicles by suitable braking apparatus. Generally, the braking apparatus takes the form of a frictional car retarder having braking bars which engage and grip the sides of the wheels of the railway vehicles. The amount of braking effort which is to be exerted on the wheels of the railway vehicles by the car retarder is dependent upon the rollability and the distance that the humped vehicle has to go to couple with the previous vehicle in the appropriated class track. In order to prevent damage to the railway vehicles upon coupling, it is necessary to ensure that the leaving speed of the vehicle from the car retarder is not excessive. That is, if the differential velocity between the coupling vehicles is greater than a given value, the ensuing collision can cause derailments and damage to the railway cars and can impair the lives of workmen and other individuals in the area. Normally, the leaving speeds of the humped vehicles is quite accurately and effectively controlled by the car retarders to permit smooth and safe coupling. However, the finds of two recent costly and tragic accidents in two different classification yards have shown that the car retarders were unable to effectively frictionally brake the wheels of certain railway cars. The two accidents involved tank cars carrying volatile hydrocarbon gases, such as propylene, propane, methane, ethane, ethylene and other hydrocarbons which were released to the atmosphere when the tankers were punctured upon collision. The leaking hydrocarbon gases were ignited by a stray spark which caused a tremendous explosion and resultant fire. One of the explosions caused more than (230) persons to be injured and resulted in (71/2) million dollars in estimated property damage. The other explosion caused an estimated (30) million dollars of property damage, resulted in approximately (300) personal injuries and caused at least one death. When the fire was extinguished and the smoke had cleared, a post-accident examination revealed that the wheels of the cars which caused the collisions were covered with a slippery foreign substance, namel, heavy grease in the first case and epoxy resin in the second case which prevented effective retardation of free rolling cars. It will be appreciated that grease, oil, and other foreign unctuous material on the wheels of humped cars cause the brake shoes of the car retarders to become ineffective in decelerating and slowing down the humped cars to a safe coupling speed. In order to positively prevent any further tank car explosions in classification yards, the Federal Railroad Administration (FRA) has invoked Emergency Order No. 5 which requires that all cars carrying explosive lading, such as, liquid petroleum gas (LPG) and the like, must be moved and classified by a locomotive or engine and that the following car in that class track must also be handled by a locomotive. The approximate annual cost of handling the explosive cars and the follow-up cars by yard locomotives is estimated at more than (55) million dollars. While the exorbitant cost of handling of explosive carrying cars by locomotives appears justified from the standpoint of protecting the life and limb of innocent people, it is obviously not the ultimate solution. It would be less costly, less time consuming and more efficient to readily ascertain the condition of wheels of the railway cars prior to humping in order to prevent possible explosions yet expedite the classification of trains. Thus, it is very desirable and highly advantageous to substantially eliminate or minimize the number of hazardous tank cars that are required to be handled by yard engines during classification by determining the coefficient of friction that will exist between the sides of the wheels of the railway cars and the brake shoes prior to humping. That is, as each of the railway vehicles approaches the hump, it is necessary to predicate whether or not the railway vehicle will be decelerated by the hump and group car retarders. If the wheels of the vehicle are clean or uncontaminated, a high coefficient of friction will exist thereby ensuring that sufficient retardation is effected to reduce the speed of the vehicles to a safe coupling speed; however, if the wheels of the vehicle are dirty or contaminated with grease, oil, paint, lading spillage or slippery foreign material, a relatively low coefficient of friction will exist thereby preventing effective retardation and resulting in the vehicle leaving the car retarder at an excessive unsafe coupling speed.